In the area of coating systems utilizing metal pigment particles, aqueous systems present rather formidable difficulties. This is particularly true with respect to aluminum and zinc pigments. Thus, the metal pigment can readily react with water to generate hydrogen gas. The amount of gas generated can produce a safety hazard, creating high pressures within the composition containers. Also, the water reaction substantially diminishes the aesthetic value of metal pigments. The reaction of aluminum pigments with water can be depicted as follows: EQU 2Al+6H.sub.2 O.dbd.2Al(OH).sub.3 +3H.sub.2 (g)
Due to the increasing demand for aqueous systems, a number of techniques have been proposed for inhibiting the attack on the pigment particles by water. Unfortunately, most of these techniques have not provided sufficient protection.
One technique that provides inhibiting properties is the passivation of the metal pigment particles with an ionic organic phosphate as disclosed by Williams et al., U.S. Pat. No. 4,565,716, the disclosure of which is incorporated herein by reference. Another technique involves the use of compounds containing hexavalent chromium or pentavalent vanadium compounds as disclosed in Kondis U.S. Pat. No. 4,693,754, the disclosure of which is incorporated herein by reference. Other techniques include the use of organic phosphites as disclosed in Kondis et al. U.S. Pat. No. 4,808,231, the disclosure of which is incorporated herein by reference, or the use of nitroparaffin solvents. Still another technique includes the use of either (a) an ionic organic phosphate compound, for example as taught in Williams et al., U.S. Pat. No. 4,565,716, or (b) a pentavalent vanadium compound, for example as taught in Kondis, U.S. Pat. No. 4,693,754, or (c) an organic phosphite compound, for example as taught in Kondis et al. U.S. Pat. No. 4,808,231, in combination with a nitro-containing solvent such as a nitroparaffin as disclosed in Keemer et al., U.S. Pat. No. 5,215,579, the disclosure of which is incorporated herein by reference.
The treated metal pigment particles can be used to form a metal pigment paste. The treatment produces a metal pigment paste which has improved gassing stability over conventionally treated products.